Electrospun polymer nanofibers for electromechanical transduction investigated by scanning probe microscopy

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Alma Mater Studiorum · Università di Bologna

Scuola di Scienze

Dipartimento di Fisica e Astronomia Corso di Laurea Magistrale in Fisica

ELECTROSPUN NANOFIBERS FOR

ELECTROMECHANICAL TRANSDUCTION

INVESTIGATED BY SCANNING PROBE

MICROSCOPY

Relatore:

Prof. Beatrice Fraboni

Correlatore:

Dott. Tobias Cramer

Presentata da:

Francesco Calavalle

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Sommario

Electrospun nanofibers for electromechanical transduction investigated by Scanning Probe Microscopy

Francesco Calavalle

Negli ultimi anni, il copolimero ferroelettrico P(VDF-TrFE), ha suscitato un grande interesse nella ricerca scientifica per le potenziali applicazioni elettroniche come ad esempio l’energy harvesting per la produzione di dispositivi indossabili e au-toalimentabili, sensori biocompatibili e memorie non volatili. Molti sforzi si sono concentrati nello sviluppo di procedure di fabbricazione che possano migliorare le performance elettromeccaniche di questi materiali. Una delle soluzioni proposte è un processo chiamato elettrofilatura, una tecnica efficiente e a basso costo che sarebbe in grado di realizzare nanofibre polimeriche già polarizzate e pronte per l’integrazione nei dispositivi.

Dalle analisi microscopiche svolte in questa tesi, utilizzando tecniche di mi-croscopia a scansione di sonda, è stato scoperto che in realtà l’elettrofilatura non provoca polarizzazione nelle fibre, bensì induce un processo di iniezione di cariche all’interno del materiale che, se testato a livello macroscopico, mostra un’apparente risposta ferroelettrica dovuta però alle cariche intrappolate, come in un elettrete. Nonostante ciò, dopo la dissipazione delle cariche spaziali, ho potuto dimostrare, grazie al’implementazione della Switching Spectroscopy PFM ad alto potenziale, che le nanofibre elettrofilate possono essere polarizzate e mostrano proprietà piezoelet-triche simili a quelle del film sottile. Quindi, inducendo la completa polarizzazione del network dopo la deposizione, è auspicabile un miglioramento delle proprietà elettromeccaniche dei dispositivi basati su nano-fibre elettrofilate.

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Abstract

Electrospun nanofibers for electromechanical transduction investigated by Scanning Probe Microscopy

Francesco Calavalle

The ferroelectric copolymer P(VDF-TrFE), has attracted, in recent years, a great interest in scientific research for modern electronics applications, such as energy harvesting for self-powered wearable devices, biomedical sensors and nonvolatile memories. Lots of efforts have been spent in the development of fabrication pro-cedures that could enhance the electromechanical performance of this material. Electrospinning has been proposed as an efficient and low-cost solution for the production of polarized polymeric nanofibers, ready for the integration in devices without post-poling process required.

In this thesis, I employ Scanning Probe Microscopy techniques to provide an accurate investigation of the electromechanical properties of single P(VDF-TrFE) electrospun nanofibers. I find that electrospinning does not induce ferroelectric polarization of the fibers, but leads to the accumulation of space charges in the material. I argument that such a space charge gives rise to an apparent ferroelec-tric response at the macroscopical scale due to the electret effect. Further, after dissipation of the space-charges, with the implementation of high voltage Switch-ing Spectroscopy-PFM, I could demonstrate that polSwitch-ing processes at the level of a single nano-fiber are possible and that the observed piezoelectric performance is comparable to the thin-film behavior. Therefore, I predict that elestrospun fibers will show improved behavior in electromechanical devices once complete poling of fiber networks is achieved.

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Introduction

An emerging direction in technological progress leads to the rapid growth of mobile and portable electronics for applications in communication, personal health care, and environmental monitoring. The integration of energy harvesting systems, ca-pable of producing energy from waste sources, is of great interest in order to project self-sustainable, wearable devices. Developments in materials science, mechanics and manufacturing now enable the construction of thin and flexible piezoelectric systems for powering electronic devices [1]. Among the wide range of piezoelectric materials, semicrystalline polymers revealed to be easily tailored at the nanoscale to achieve non-toxic, low-temperature, and relatively low-cost processing structures with a high flexibility, light weight, and easy deformation properties. For all these advantages, they have recently raised great interest in the scientific community, offering new and promising solutions for the fabrication of polymer-based devices. In recent years, intensive research efforts has been made on the studies of new organic devices based on polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (TrFE), which are the most representative organic ferroelectric, and thus piezoelectric, materials. The FE/PE properties of these polymers arise due to the molecular dipoles present in the monomer unit (CH2-CF2) that are

aligned perpendicular to the main chain axis [2]. Because of the ferroelectric prop-erties, P(VDF-TrFE) is not only attractive for electromechanical application, but it is also promising candidate for the next generation nonvolatile high-density mem-ory applications which can replace perovskite ceramics currently used in the com-mercial ferroelectric random access memories (FRAMs) [3, 4]. Furthermore, due to their biocompatibility, these polymers are suitable for biomedical application such as implantable sensors [5]. Despite the great advantages described, generally, P(VDF-TrFE) presents lower performance compared to more traditional ceramic FE/PE materials such as PZT and BTO [6].

Preparation process optimizations, and dimension lowering strategies have pre-sented from previous work as the most effective way to improve the performance of polymer-based devices. Nanostructured materials, exhibit new physical phenomena

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because of finite size effects that occur during down-scaling [7,8]. Electrospinning is one of the technique proposed for the low-cost production of nanofibers from polymeric solutions. It has been suggested that this method can give rise to pref-erentially oriented induced dipoles in P(VDF-TrFE) nanofibers with a single stage process [9].

Although, some improvements in performance of devices with integrated P(VDF-TrFE) electrospun nanofibers has been observed, it is not clear what relationship there is between the fabbrication method and the microscopic effects on properties of P(VDF-TrFE).

The objective of this thesis is to investigate the effect brought by electrospin-ning process on ferroelectric and piezoelectric properties of P(VDF-TrFE) at the nanometer scale, using Scanning Probe Microscopy techniques.

Chapter 1, starts with a wide introduction on the origin and the theory be-hind ferroelectricity and piezoelectricity in dieletric materials. In addiction, the structure and the main properties, as well as the most attractive applications of P(VDF-TrFE) are presented.

P(VDF-TrFE) thin film and electrospun nanofibers samples, analysed in this work, are described together with the respectively fabrication methods in Chap-ter 2.

The next Chapter provides to explain PFM (Piezoresponse Force Microscopy) and KPFM (Kelvin Probe Force Microscopy), the two SPM techniques used to perform measurements on the samples. These methods of investigation are funda-mental for the purpose of my work, since they allow the acquisition of information about piezoelectric response and surface potential of the samples with an extraor-dinary spatial resolution.

In Chapter 4, the results collected by the experiments designed during this work are described and discussed. The Chapter starts with the description of the setup implemented for Switching Spectroscopy PFM measurements. In the central part, the results obtained from PFM measurements on P(VDF-TrFE) thin film and electrospun nanofibers are compared. Finally, surface potential measurements are reported to explain the charging process induced by electrospinning.

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Chapter 1 Piezo- and Ferroelectric polymers

in electronic applications

Piezoelectricity was observed for the first time in 1880 by French physicists Jacques and Pierre Curie. A subset of piezoelectricity is ferroelectricity, so all ferroelectric materials are piezoelectric (Figure1.1). The term ferroelectricity arise by analogy with ferromagnetism, mainly because they have similar characteristics respectively related to electric and magnetic field. The prefix piezo- in the word piezoelectrics intead is derived from a Greek word, piezein, meaning pressure. Piezoelectrics are materials in which electricity can be generated by an applied mechanical stress or a mechanical stress can be produced by an applied electric field. The term piezoelectricity has been used by scientists since 1881 to distinguish the piezoelec-tric phenomena from electrospiezoelec-triction. Piezo- and ferroelecpiezoelec-tric materials exhibit interesting electromechanical properties that have led to interesting applications. There is, therefore, a need to have a basic understanding of piezo- and ferroelectric materials as well as the relationship between them when developing piezoelectric-based energy harvesting systems, ferroelectric memories or other sensoring appli-cation.[10]

The aims of this chapter is to give the reader a basic introduction to the un-derlying physics and principles of piezo- and ferroelectric materials. These are materials that have been well known and described since the late 1800s and as such there is significant historical knowledge and understanding to be drawn upon. Furthermore, it will be described a wide range of application in modern electronics, such as energy harvesting and nonvolatile memory.

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1.1 Background

1.1.1 Dielectric materials

Figure 1.1: _{Relationshinp between crystal classes and piezo-,pyro- and ferroelectic} properties [11].

In general, piezo- and ferroelectric materials are first of all dielectrics, in fact the characteristic properties of these materials are strongly related to the polar-ization induced by external electric fields or other stimuli like mechanical stress. All dielectric materials when subjected to an external electric field exhibit a dis-placement of cations and anions respectively in the same and opposite direction of the applied field, resulting in a net deformation of the material. The degree of deformation depends on crystal class to which the dielectric belongs. Of the 32 crystal classes, 11 are centrosymmetric (i.e., possess a centre of symmetry or inver-sion centre) and 21 are noncentrosymmetric (do not possess a centre of symmetry) [11]. When a dielectric material possessing a centre of symmetry is subjected to an external electric field, due to the symmetry (inversion centre), the movements of cations and anions are such that the extension and contraction get cancelled between neighbouring chemical bonds and the net deformation in the crystal is ideally nil. But the chemical bonds are not perfectly harmonic and, due to the anharmonicity of the bonds, there will be second-order effects resulting in a small net deformation of the lattice. This effect is called electrostrictive effect and exists in all dielectrics caused by the anharmonicity of chemical bonds. When a dielectric material belonging to a noncentrosymmetric class (except the octahedral class)

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1.1. Background 5 is subjected to an external electric field, there will be asymmetric movement of the neighbouring ions, resulting in significant deformation of the crystal and the deformation is directly proportional to the applied electric field. These materials exhibit an electrostrictive effect due to the anharmonicity of the bonds, but it is masked by the more significant asymmetric displacement. This materials are called piezoelectric materials [11]. The classification of dielectric materials based on their response to external stimuli is shown in Figure1.1.

1.1.2 Electric Polarization

In an atom or a molecule when the centres of positive and negative charges are separated by a certain distance d, the atom or the molecule possesses an electric dipole moment given by

p = qd, (1.1)

where q is the charge, d is the separation between the positive and negative charge centres and p is a vector with direction from negative to positive charge. Dielectric materials may be classified as polar and nonpolar. In nonpolar dielectric materials, normally the atoms do not possess an electric dipole moment as the centres of positive and negative charges coincide. When these materials are subjected to an external electric field, the centres of positive and negative charges get separated and thus dipole moments are induced untill the field is applied. In polar dielectric materials, each atom or molecule possesses a dipole moment as the centres of positive and negative charges do not coincide, so when an external electric field is applied to these materials, the electric dipoles tend to orient themselves in the direction of the field. Examples of polar and nonpolar materials are respectively H2O and O2 [11].

A polar dielectric material consists of a large number of atoms or molecules each possessing an electric dipole moment. Electric polarization P is defined as the total dipole moment per unit volume and is given by

P =

P

i pi

V , (1.2)

where V is the volume of the material. P is a vector normal to the surface of the material, and it is sometimes called the surface charge density. In general, in a polar dielectric, the individual electric dipoles are all randomly oriented and so the

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net polarization is zero. When an electric field E is applied, the material develops a finite polarization that increases as the field increase, reaching saturation when all the dipole moments are oriented in the direction of the field.

The displacement field D developed inside the material due to the external field is given by

D = ǫ0E + P, (1.3)

where ǫ0 is the permittivity of free space. D is also expressed by the relation

D = ǫE = ǫ0ǫrE, (1.4)

where ǫr is the relative permittivity or dielectric constant of the dielectric material.

The polarization P is directly related to E by the relation

P = ǫ0χE, (1.5)

where χ is called the electric susceptibility of the material [11]. From Equations

1.3, 1.4 and 1.5, we get the relation between the dielectric constant and electric susceptibility as

ǫr = 1 + χ. (1.6)

1.1.3 Electrostrictive effect

Electrostriction is a basic electromechanical phenomenon that occurs in all insu-lators or dielectrics. It is present in all crystal symmetries because is a fourth rank polar tensor. Electrostriction is a measure of the polarization induced by ions shifting away from their natural equilibrium positions, giving rise to variations in the lattice parameters (strain). In centrosymmetric crystals, the induced shifts of equivalent ions almost cancel each other out, while the difference in the shifts because of potential anharmonicity generates strain. The induced strain (Sij) is

proportional to the square of electric field (Ei)/polarization (Pi), and it can be

expressed in the following equations:

Sij = QijklPkPl,

Sij = MijklEkEl,

(1.7)

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1.2. Theory of Piezo- and Ferroelectric effect 7 The strain induced by the electrostrictive effect is generally small when com-pared with that induced by piezoelectricity. Consequently, there has been limited attention focusing on the electrostrictive effect. In the 1980s, a systematic study on electrostriction was carried out on relaxor ferroelectrics with perovskite structures in which a high electrostrictive strain was observed because of the high dielectric response of the relaxors [12]. In addition, in the 1990s, investigations on elec-trostriction were performed on polymers, including ferroelectric polymers, dielec-tric elastomers, and polymer composites. Ultra-high electrosdielec-trictive strains were observed in these polymeric materials (>4% for polyvinylidene fluoride [PVDF] and >40% for silicone), giving them potential for use in actuator applications [13]. On the other hand, ferroelectrics are the mainstay materials for piezoelectric trans-ducer and actuator applications and have been reported to possess much higher piezoelectric responses when compared with non-ferroelectric materials [14]. The electrostrictive effect plays a key role in the electromechanical behavior in ferro-electrics, investigations on which will benefit the exploration of high-performance piezoelectrics [15].

1.2 Theory of Piezo- and Ferroelectric effect

1.2.1 Piezoelectricity

Dielectric materials that belong to the class of noncentrosymmetric crystals (Fig-ure 1.2) are classified as piezoelectric materials. There’s a lot of materials that ex-hibit piezoelectricity, for examples natural like Quartz and Lead titanate (PbTiO3)

and synthetic ones like quartz like crystals, perovskite ceramics and also polymers. To achieve the piezoelectric properties usually these materials are subjected to a "poling process" in order to align the domains of polarization in a preferential direction.

When these materials are subjected to external strain by applying a mechanical sollecitation like strain or pressure, the electric dipoles in the crystal get oriented such that the crystal develops positive and negative charges on opposite faces, re-sulting in an electric field across the crystal. This is called direct piezoelectric effect. An interesting feature of this phenomenon is the reversibility, in fact in the opposite situation the application of an external electric field, will result in an asymmetric displacements of anions and cations that cause considerable net deformation of

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Figure 1.2: Typical crystal structure of a piezoelectric perovskite, before and after polarization [16].

the crystal. This is known as the indirect piezoelectric effect. The relation be-tween strain and applied electric field is, in first approximation linear, unlike for electrostrictive materials in which the strain is proportional to E2

. The strain in a piezoelectric material is extensive or compressive, depending on the polarity of the applied field. The direct and indirect piezoelectric effects are illustrated in Figure1.3. In the direct effect when a poled piezoelectric material is subjected to tensile stress, in the direction parallel to the poling direction, a positive voltage is generated across the faces. When the stress is compressive in the same direction, a negative voltage is generated across the faces. In the indirect effect, when an external voltage is applied to the material, the material gets extended if the polar-ity of the voltage is the same as that of the field applied during poling and, when the voltage is applied in the reverse direction, the material gets compressed. The reaction generated by the applied stimulus in both case is barely instantaneous, so the application of external alternating fields produces alternate compression and extension at the same frequency of the electric field.

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1.2. Theory of Piezo- and Ferroelectric effect 9 Now let’s introduce the equations which describe electromechanical properties of piezoelectric materials. In this formulation it’s been assumed that piezoelectric materials are linear, this is a good approximation at low electric fields and at low mechanical stress levels. However, they may show considerable nonlinearity if operated under a high electric field or high mechanical stress level. A tensor notation is adopted to identify the coupling between the various entities through the mechanical and electrical coefficients. The common practice is to label directions as depicted in Figure 1.4, so with 1, 2, 3 to indicate x, y, z and 4, 5, 6 to indicate the shear planes perpendicular to the respective axis.

Figure 1.4: Tensor direction for defining constitutive equation [18].

Piezoelectricity can be described as a coupling between behaviour of elastic variables, stress T and strain S, and dielectric variables, displacement of charge density D and applied electric field E. Combining Equation 1.4 and the Hook’s Law:

S = sT, (1.8)

where s is the compliance, one can obtain the electromechanical equations for a linear piezoelectric material:

Si = sEijTj + dmiEm, (1.9)

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where the index i, j = 1, 2, 3, 4, 5, 6 and m, k = 1, 2, 3 refer to different directions within the material coordinate system, d is the matrix of piezoelectric constants, ǫ is the dielectric permitivity and the superscripts T and E indicates constant electric field and stress field across the system [17]. There a total of four piezoelectric coef-ficients, dij, eij, gij, hij, all related to each other trough electrical and mechanical

properties of the material and by convention are defined as follows:

dij = ∂Di ∂Tj !E = ∂Sj ∂Ei !T , (1.11) eij = ∂Di ∂Sj !E = ∂Tj ∂Ei !S , (1.12) gij = ∂Ei ∂Tj !D = ∂Sj ∂Di !T , (1.13) hij = ∂Ei ∂Sj !D = ∂Tj ∂Di !S . (1.14)

In each Equation above the right terms are related to the direct effect and the left ones to the inverse effect; dij represent the piezoelectric strain constants,

eij the stress constants, gij the piezoelectric voltage constants and hij the strain

constants [18]. Equations 1.9, 1.10 could have been written in the matrix form applying some simplifications. In fact if one assumes the device or material trans-versely isotropic and that it’s poled along one axis (for example axis 3), many of the parameters in matrix will be either zero or can be express in terms of other parameters. Rewriting those equations in matrix form one obtaines the following expression:               S1 S2 S3 S4 S5 S6               =               s11 s12 s13 0 0 0 s12 s11 s13 0 0 0 s13 s13 s33 0 0 0 0 0 0 s44 0 0 0 0 0 0 s44 0 0 0 0 0 0 2(s11− s12)                             T1 T2 T3 T4 T5 T6               +               0 0 d31 0 0 d31 0 0 d33 0 d15 0 d15 0 0 0 0 0                    E1 E2 E3      , (1.15)

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1.2. Theory of Piezo- and Ferroelectric effect 11 and      D1 D2 D3      =      0 0 0 0 d15 0 0 0 0 d15 0 0 d31 d31 d33 0 0 0                    T1 T2 T3 T4 T5 T6               +      ǫT 11 0 0 0 ǫT 11 0 0 0 ǫT 33           E1 E2 E3      . (1.16)

The “piezoelectric strain constant” d is defined as the ratio of developed free strain to the applied electric field. The subscript dij implies that the electric field

is applied or charge is collected in the i direction for a displacement or force in the j direction. In order to give a physical explanation of the d coefficient let’s consider a device made of two electrodes connected to opposite sides of a piece of piezoelectric material poled in direction 3 of thickness t, length l and width w. If a voltage V is applied to this transducer this voltage generates the electric field:

E3 =

V

t , (1.17)

which strains the transducer in different directions. In particular

S1 = ∆l l , (1.18) in which ∆l = d31V l t . (1.19)

The piezoelectric constant d31 is usually a negative number. This is due to the

fact that application of a positive electric field will generate a positive strain in direction 3. Another interpretation of dij is the ratio of short circuit charge per

unit area flowing between connected electrodes perpendicular to the j direction to the stress applied in the i direction. If a force F is applied to the transducer, in the 3 direction, it generates the stress

T3 =

F

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which results in the electric charge

q = d33F, (1.21)

flowing through the short circuit [17]. The other coefficients are of less interest for the goal of this work but they can be treated with analogous considerations and they give other relationship between electromechanical properties.

Piezoelectric Coupling Coefficient k

Piezoelectric coupling coefficient is a measure of the efficiency of a piezoelectric material as a transducer. It quantifies the ability of the piezoelectric material to convert one form of energy (mechanical or electrical) to the other form (electrical or mechanical). If one define the mechanical work made by an applied force WM

as

WM =

F ∆z

2 , (1.22)

and the elecrical energy WE generated by the displacement of charge due to the

piezoelectric effect

WE =

Q2

2Cp

, (1.23)

which is the energy stored in the piezoelectric capacitor. If one consider directions parallel to the polarization the piezoelectric couping will be

k33= s WE WM = q Q F ∆zCp . (1.24)

In general this coefficient can be written in terms of other parameters and direction, in particular as k2 ij = d2 ij sE ijǫTij . (1.25)

1.2.2 Ferroelectricity

Ferroelectric materials are a subclass of piezoelectric materials. Thus, they exhibit piezoelectric properties and also they have more sensitive characteristics, so most of the practical piezoelectric devices use ferroelectric materials. Ferroelectric materi-als exhibit spontaneous polarization but they have the characteristic property that allows an inversion of this polarization with he application of an external electric

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1.2. Theory of Piezo- and Ferroelectric effect 13 field. another feature of ferroelectrics is that the polarization P as a function of the applied electric field E is nonlinear. The main properties of ferroelectric materials are the following:

• Ferroelectric hysteresis; • Spontaneous polarization; • Reversible polarization;

• Ferroelectric phase transition temperature.

The description of the arrangement of polarization in a ferroelectric material uses a domain structure similar to the structure used in ferromagnetic materials. Ferroelectric domain is defined as a small microscopic region in the material within which all the electric dipoles are oriented in the same direction due to a strong short-range interaction caused by internal electric fields. This preferential direction of polarization is present within the domains even without applied electric field and it is called spontaneous polarization. Ferroelectric materials are made of a large number of domains, each one with a spontaneous polarization in a specific direction. Normally, the domains are all randomly oriented, and so the net polarization of the material is zero in the absence of an external electric field. When an external electric field is applied, the domains tend to get oriented in the direction of the applied field. The process that regulate this reorientation consists of the growing in size of domains that are in the direction of the external field at the expense of the other domains. As the external field is increased, more and more domains get oriented in the direction and ultimately the material ideally consists of a single domain [11].

Phase transitions

The properties of ferroelectrics can be understood by reference to a fictitious one-dimensional crystal made up of two atoms of opposite charge. In this crystal, it is clear that we can orient the dipoles to point all to the right, or all to the left. The two structures are completely equivalent, except that they have an opposite sign to the dipole moment. They must therefore have exactly the same energy. We could transform one into the other by dragging one type of atom toward the other. As we do this, the bulk polarisation will reduce in magnitude, and change

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Figure 1.5: Schematic potential wells and crystal configuration of two ferroelectric states [19].

sign at the point where the atoms are equally spaced and finally switch to the opposite direction. Given that we know the crystal is stable in either of the two polarised states, there must be an energy barrier between the two states, and we can sketch a curve (Figure 1.5) for the energy as a function of the polarisation. It is common to observe that as the temperature is raised, the bulk polarisation decreases and vanishes abruptly at a temperature Tc. This is a phase transition,

just as in a ferromagnet raised above its Curie temperature, or a solid raised above its melting point. It arises microscopically because as temperature is raised the thermal vibrations of the atoms in the solid cause flctuations which overcome the potential barrier between the two (or more) wells.

The detailed microscopic theory of phase transition will be different from mate-rial to matemate-rial, but the macroscopic properties will be similar across many classes of materials and they can be described with the Landau theory of phase transitions [20].

Ferroelectric hysteresis Loop

The application of an external electric field to switch polarization in a ferroelectric material leads to the characteristic hysteresis loop of P vs E. With the application of an electric field E the two stable states no longer have the same energy because of the electric polarisation energy −P·E, so the wells are tilted by the electric field. It is also clear from Figure1.6 that a small field will not necessarily immediately switch the polarisation from one direction to the other because there is a barrier to be overcome.

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1.2. Theory of Piezo- and Ferroelectric effect 15

Figure 1.6: Typical hysteresis loop of a ferroelectric material.

Initially, when the applied field is zero, the ferroelectric domains are all ran-domly oriented and so the polarization is zero. As the field is increased, the domains get oriented in the direction of the field, and the polarization increases linearly in the beginning. As the field is further increased, more and more domains get oriented, the curve becomes nonlinear, and ultimately when all the domains get oriented, the polarization attains the maximum value PS called saturation

polar-ization. If the electric field is now reduced gradually, the polarization decreases but the curve is not retraced. The decrease in polarization is rather slow because of the barrier for the reorientation of domains and the polarization lags behind the electric field. When the field is reduced to zero, there remains a finite polarization called the remnant polarization PR. In order to make the remnant polarization

disappear, an electric field in the reverse direction has to be applied. At an electric field of −EC called the coercive field, the polarization becomes zero. If the field

is further increased in the reverse direction beyond EC, the domains get oriented

in the direction of the field and the polarization increases with increasing field (in the new reversed direction) untill saturation polarization (−PS) is reached in the

reverse direction. If the field is now again reduced back to zero, there will be again remnant polarization and increasing the field from zero in the positive direction, the remnant polarization disappears when the field is +EC. Further increase in the

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1.3 Electrets

A class of dielectrics that it hasn’t been mentioned yet are the electres, which properties are often related to piezoelectric and pyrolectric materials. An electret is a piece of dielectric material exhibiting a quasi-permanent electrical charge (Fig-ure 1.7). The electret charge may consist of "real" charges, such as surface-charge layers or space charges; it may be a "true" polarization; or it may be a combina-tion of these. While the true polarizacombina-tion is usually a frozen alignment of dipoles, the real charges comprise layers of trapped positive and negative carriers, often positioned at or near the two surfaces of the dielectric, respectively. The electret charges may also consist of carriers displaced within molecular or domain struc-tures throughout the solid, resembling a true dipole polarization. On metallized electrets, a compensation charge may reside on the electrode, unable to cross the energy barrier between metal and dielectric. Mostly, the net charge on an electret is zero or close to zero and its fields are due to charge separation and not caused by a net charge. An electret not covered by metal electrodes produces an external electrostatic field if its polarization and real charges do not compensate each other everywhere in the dielectric. However, as Heaviside already realized in 1892 [21], the fields of an electret may be compensated within a short time period by the relative motion of real charges and dipoles. This is observed in many piezoelectric substances, so usually the definition of electrets includes also some piezoelectric materials [22].

Figure 1.7: Schematic cross section of a one-sided metailized electret having deposited surface charges, injected space charges, aligned dipolar charges and compensation charges

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1.3. Electrets 17

1.3.1 Charge formation in electrets

In his early works on electrets, Eguchi [23] found that freshly prepared electrets from wax mixtures, just after removal from the high-voltage electrode assembly, exhibited charges which had sign opposite to that of forming electrodes. That is, the surface which had been in contact with the anode showed a strong negative charge, and the cathode showed positive charge. These initial charges, were found to decay within a few hours or days after the preparation of the electrets. Subsequently they passed through zero and assumed a sign equal to that of the corresponding forming electrode. This type of charge would reach a maximum and then gradually decay with time. This charge was found to be somewhat permanent for all practical purposes, lasting for several years when used for practical applications. Andrew Gemant [24] named the former type of charge “heterocharge” and the latter type of charge “homocharge". Later, Gross [25] gave the well known “two-charge theory” to explain the behavior of an electret.

Two-Charge Theory

Gross treats electrets from the point of view of absorptive dielectrics. He assumes a decay of electrification caused partly by external and partly by internal conduction within the dielectric itself. The dielectric absorption related to the movement of ions or the orientation of dipoles in the interior of the dielectric gives rise to hete-rocharge, while the conduction in the dielectric-electrode interface which produces the homocharge. In polar materials, the formation of heterocharge is due mainly to the orientation of the dipoles. With sufficiently high field strengths, conduction currents surge into the interface and consequently electrons are fed into the dielec-tric or extend from its surface and are transferred to the electrodes. This facilitates the formation of homocharges, which appear in the form of surface charges, later spraying over a certain depth within the dielectric. Because of this process, a weak-ening of the field occurs which reduces the conduction currents. With short-circuit, a part of the dielectric polarization disappears immediately, and the rest follows more or less slowly. Due to this decay of the heterocharge, the homocharge compo-nent begins to prevail in the resulting field. Again, the conduction currents play a role and cause the decay of homocharge, but there is no recombination of these two types of charges [25]. Thus Gross concludes that there exist two types of charges of opposite signs in a short-circuited dielectric. The heterocharge is associated with

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dielectric absorption which is due to a homogeneous volume polarization of the dielectric. Homocharge is due mainly to the surface breakdown, which is purely an ionic surface charge. Thus the two-charge theory is capable of explaining the coexistence of the heterocharge and homocharge in a polarized sample and most of the experimentally observed results [26].

1.4 Piezo- and Ferroelectric polymers

By 1927 it was well understood that molecules containing permanent electric mo-ments orient in the direction of an electric field when mobile in the liquid state. Upon solidification of the material in the presence of the field, the dipoles lose their mobility while retaining their preferred orientation. The net dipole orientation pro-duces the electret’s permanent polarization (net dipole moment per unit volume). It was also recognized that in addition to the electret’s moment there were real charges, generally concentrated near the electret surfaces, which were injected dur-ing the formation process by field emission, gas breakdown or similar processes. In 1927 piezoelectricity and pyroelectricity were shown theoretically and experi-mentally to be properties exhibited by electrets with preferentially ordered dipoles [27]. However, these early wax electrets had poor mechanical strength and low sen-sitivity, and applications for them did not develop. More recently, strong, highly active polymer films, notably poly(vinylidenefluoride), PVDF, poly(vinylfluoride), PVF, and poly(vinylchloride), PVC, have been recognized for their potential value as thermoelectric and electromechanical transducer materials. For this reason a lot of work has been done on production of different kind of synthetic polymers, amor-phous and semicrystalline [22]. From the first studies on this class of polymers, was clear that semicrystalline polymers were more promising for potential applica-tions. In particular, in 1969, strong piezoelectricity was observed in PVDF, with the piezoelectric coefficient of poled thin films as large 10 times larger than that observed in any other polymer [28]. Another interesting thing is that at high elec-tric fields, the polarization that occurs in polymers such as PVDF is nonlinear with the applied electric field. This nonlinearity in polarization is defined as hysteresis. The existence of a spontaneous polarization together with polarization reversal is generally accepted as proof of ferroelectricity. Although ferroelectric phenomenon has been well documented in ceramic crystals, the question of whether polymer

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1.4. Piezo- and Ferroelectric polymers 19 crystallites could exhibit dipole switching was debatable for about a decade follow-ing the discovery of piezoelectricity in PVDF. Inhomogeneous polarization through the film thickness which yielded higher polarization on the positive electrode side of the polymer led to speculations that PVDF was simply a trapped charge elec-tret. These speculations where dispelled when X-ray studies [29] demonstrated that polarization anisotropy vanishes with high poling field strengths and that true ferroelectric dipole reorientation occurs in PVDF. However, is still a subject of study the role of trapped charges in the polarization stability and orientation. In recent years great improvements has been done on comprehension and realization of this polymer and its related copolymers. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers and are currently the only commercially available piezoelectric polymers. In the next sections, it will be given a description of PVDF and its copolymer P(VDF-TrFE) that are the most repre-sentative organic ferroelectric polymers and also the subject of this work.

1.4.1 PVDF

PVDF (Poly(vinylidene fluoride)) has a very simple chemical formula, -CH2-CF2

-, intermediate between polyethylene (PE) -CH2-CH2-, and

polytetrafluoroethy-lene (PTFE)-CF2-CF2. The spatially symmetrical disposition of the hydrogen and

fluorine atoms along the polymer chain gives rise to unique polarity effects that influence the electromechanical response, solubility, dielectric properties, crystal morphology and yield an unusually high dielectric constant. The dielectric con-stant of PVDF is about 12, which is four times greater than most polymers, and makes PVDF attractive for integration into devices as the signal to noise ratio is less for higher dielectric materials. The amorphous phase in PVDF has a glass transition that is well below room temperature (-35 ◦_{C), hence the material is}

quite flexible and readily strained at room temperature. Because of these struc-tural characteristics, PVDF takes many types of molecular and crystal structures, which change depending on the preparation conditions of the samples. The PVDF polymer is typically 50% to 60% crystalline depending on thermal and processing history and has five different phases, called the α, β, γ, δ and ǫ phases [30], among which the α phase is nonpolar and the β phase is the most polar (Figure 1.8). The most stable, non-polar α phase results upon casting PVDF from the melt and

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can be transformed into one of the polar phases by mechanically stretching at el-evated temperatures or by rotating the molecular chain axis with a high electric field. The β phase is most important for piezoelectric considerations beacuse has a dipole moment perpendicular to the chain axis.

Figure 1.8: Representation of PVDF molecules; on the left it is shown the structure of principal crystal phases of PVDF, on the right the changing in structural formula of copolymer PVDF-TrFE [30].

1.4.2 P(VDF-TrFE)

The copolymer P(VDF–TrFE) (poly(vinylidene fluoride-trifluoroethylene)) is of great interest beacuse crystallizes predominantly in the β phase, readily showing an intrinsic polarization, in fact it has been shown that exhibit strong piezoelec-tric, pyroelectric and ferroelectric effects. However, because of the semicrystalline nature of these polymers, PVDF–TrFE can also present a paraelectric phase, even after polymer annealing. The structure of this copolymer, as shown in Figure 1.8, is practically the same of PVDF, with the insertion of PTrFE between consecutive PVDF fundamental units. An attractive morphological feature of the comonomers is that they force the polymer into an all-trans conformation that has a polar crys-talline phase, which eliminates the need for mechanical stretching to yield a polar phase. P(VDF-TrFE) crystallizes to a much greater extent than PVDF (up to 90 % crystalline) yielding a higher remnant polarization, lower coercive field and much sharper hysteresis loops. TrFE also extends the use temperature by about

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1.5. Applications 21 twenty degrees, to close to 100◦_{C [}₁₈_{]. Although the piezoelectric constants for the}

copolymers are not as large as the homopolymer, the advantages of P(VDF-TrFE) associated with processability, enhanced crystallinity, and higher use temperature make it favorable for applications. The piezoelectric constants d33 for highly

or-dered lamellar P(VDF-TrFE) have recently reached values of about -38 pm/V [31]. Recently, it was shown that geometrical confinement, which leads to a high-aspect-ratio PVDF and P(VDF–TrFE) nanostructures with at least one feature size below 100 nm, could have a profound influence on the final piezoelectric performances of these macromolecules [32]. In particular, preferential crystallization in the polar

β phase was shown not only in P(VDF–TrFE) nanostructures but also in PVDF

nanowire arrays, and it led to remarkable levels of polarization [33] without further processing the polymer with mechanical stretching or electrical poling, in contrast to the polymer’s conventional performance in bulk or thin films. In the next Chap-ters it will be presented a technique for the production of thin films and nanofibers and a further investigation about the effects of nanoconfinement.

1.5 Applications

In recent years, driven by the rapidly developing miniaturized electronics, new or-ganic devices based on polyvinylidene fluoride (PVDF) and its copolymer with tri-fluoroethylene (TrFE) have attracted intensive research interest. The great piezo-, pyro- and ferroelectric properties showed by these materials have made them the most promising candidates for a new generation of devices for energy harvesting, nonvolatile memories and sensors. Their attractive advantages include low tem-perature processing, low-cost solution processing, outstanding chemical stability, and non-toxicity [34]. Previous studies showed how the most effective methods of improving the performance of these materials consist in doping modifications, preparation process optimizations, and dimension lowering strategies. Among this class of materials, low-dimension ferroelectric materials have become widely stud-ied within the field ferroelectric materials research because of the new physical properties and phenomena caused by the reduction of dimensions at nanoscale. The discovery of Langmuir-Blodgett (LB) ferroelectric polymer films in 1995 [35] led to investigation of ferroelectric properties and finite size effect at the nanoscale. Since then, it has been a subject of intensive study not only in theoretical aspects

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but also in technical applications. There have been strong debates of the theoret-ical models used to explain the experiments. Currently, the explanations for the switching behavior in P(VDF-TrFE) thin films are still controversial [36]. Recently, one-dimensional (1D) ferroelectric nanostructures (wires, rods, tubes, belts, and fi bers) have been extensively studied because of their specific ferroelectric behaviors related to 1D morphologies. Generally, the specific properties of 1D ferroelectric nanostructures are attributed to the increased surface area. Moreover, decrease in size and dimensionality can facilitate the formation of single domain structures, which can dramatically enhance ferroelectric properties. Therefore, 1D ferroelec-tric nanostructures present great potential for use in nonvolatile memory devices, microelectromechanical systems, FE-PV devices, nonlinear optics, nanogenerators, and sensors. In recent years, considerable progress has been achieved in the study of 1D nanostructured ferroelectrics (e.g., synthesis, properties, and applications), although barriers to the practical application of nanodevices remain [37]. In this section it follows a brief review of some of the most interesting and cited application based on nanostructured PVDF polymers.

1.5.1 Energy harvesting devices

Figure 1.9: a) Possible sources of energy for harvesting (left) and opportunities use of this energy in sensing and actuation (right) that can be considered for flexible/bendable piezoelectric devices [38]. b) Example of a P(VDF-TrFE) naofibers-based device [39].

Energy harvesting is the process by which energy is derived from external sources (e.g. ambient energy), captured, and stored for small, wireless autonomous devices, like those used in wearable electronics and wireless sensor networks. Among these energy sources, mechanical energy may be the most widely distributed and is specialized for human motion-related applications. It exists abundantly as different

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1.5. Applications 23 forms and is frequently located in our local environment, but the vast majority is ignored and wasted, such as human motion, walking, mechanical triggering, vibra-tion, wind, flowing water and so forth (Figure 1.9a). An outstanding feature of nanogenerators technology is its simplistic and diverse structural properties that can be applied for flexible and stretchable electronics. Flexible electronics are attracting substantial attention because of their promising applications in many areas, such as wearable electronics and bendable displays. The realization of fully flexible electronics demands to have an appropriate flexible power supply device. P(VDF-TrFE) is one of the most interesting materials to create such devices and is commonly used for piezoelectric applications because of his advantageous properties of flexibility, adequate mechanical strength, ease of processing and high chemical resistance. Polymers’ chemical stability and biocompatibility is in favor of its ap-plication in biological systems, such as implantable sensing and energy harvesting. A notable disadvantage is that achieving good performance requires electrical pol-ing in which mechanical stretchpol-ing processes need to align the dipoles of the polar

β-phase of PVDF structures [40]. During the last decade different kind of devices has been developed by research, most of them were based on thin film structures. Recently it has been demonstrated that better performance can be reached using electrospun P(VDF-TrFE) nanofibers as shown in Figure 1.9b [39].

1.5.2 Nonvolatile Memories

Figure 1.10: Schematic view and operating mechanism of a back-gate FeFET-based nonvolatile memory device [41].

Another important application of P(VDF-TrFE) thin films and fibers is ferro-electric nonvolatile memory (Figure1.10). The principle of nonvolatile ferroelectric random access memories (FRAMs) is based on the polarization reversal by an ex-ternal applied electric field. The binary logic states “1” and “0” are represented

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by the nonvolatile storage of the positive or negative remanent polarization states. The nonvolatile property is due to the fact that the sample can hold the polar-ization state when the external field is removed. Compared to other nonvolatile memories, for example, flash, electrically erasable and programmable read-only memories (EEPROM) FRAMs have faster write and read times, lower power con-sumption, and high write and read endurance. FRAMs can be applied in a variety of consumer products, such as smart cards, power meters, printers, videogames, and radio-frequency identification (RFID) tags. The available commercial FRAMs are based on perovskite-type ferroelectrics such as lead zirconium titanate (PZT) but generally require high annealing temperature (>400 ◦_{C), which is harmful to}

other components on the chip. P(VDF-TrFE) copolymer is the promising material to replace perovskites for his already cited properties regarding easy processing, low temperature, low-cost and chemical stability [36]. Although there is advantage in using P(VDF-TrFE) for FRAMs, there are also some practical challenge to over-come in order to commercialize such memory devices. First, an improvement in performance, in fact lots of groups reported a problem of high operational voltage [42]. Second, an appropriate approach must be found to control switching dynam-ics in the copolymer films. Finally, the solution processing has to be incorporated into the semicondutctor-manifacturing process.

1.5.3 Other Applications

Figure 1.11: a)Example of ferroelectric transistor Fe-FET [43], b) structure of polymer photovoltaic devices with FE interfacial layers and a schematic diagram of electric field. The right-hand panel illustrates the electric-field distribution and electron conduction through the P(VDF-TrFE) on the Al side [44].

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1.5. Applications 25

Negative Capacitance devices

Conventional field-effect transistors (FETs) require a change in the channel poten-tial of at least 60mV at room temperature to induce a change in the current by a factor of 10, which is determined by the Boltzmann limit. Recently the concept of coupling the ferroelectric layer to the channel of a field effect transistor for lowering the subthreshold swing due to the negative capacitance effect is actively studied [36]. This concept is proposed by Salahuddin and Datta in 2008 [45] which opens a new route for the realization of transistors with steeper subthreshold characteris-tics and thus enabling low power dissipation. The experimental validity of Fe-FET with a sub-60mV/decade switching behavior is firstly demonstrated incorporating a thin P(VDF-TrFE) film into a gate stack. Figure 1.11a shows the device con-figuration of the negative capacitance FET [43]. The challenges include successful integration of ferroelectric/dielectric gate stacks onto FET structures especially for oxides and efficient designs to ensure hysteresis free operation. More efforts need to be spent to include temperature and interface effects on the device performance.

Photovoltaic devices

Recently it is reported that, by using a permanent electric field of an ultrathin layer of ferroelectric P(VDF-TrFE) introduced at the interface between the electrode and a semiconductor layer in an organic photovoltaic (OPV) device, the charge pair separation and charge extraction efficiency can be increased and thus the power conversion efficiency (PCE) is enhanced by up to 200% [44]. OPV devices have been intensively investigated during the last few years due to their promising application for future low cost and high performance energy sources. The energy loss in OPV devices is mainly caused by the recombination of electrons and holes in semiconducting polymer-fullerene blends. To separate the electrons and holes and prevent their recombination by an external field is essential to increase the OPV efficiency [36]. A large internal electric field can be induced incorporating a ferroelectric layer thus eliminating the need for an external field. The device structure and working principle of the OPV based on a P(VDF-TrFE) thin film are illustrated in Figure 1.11b.

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Sensors and actuators

Obviously the electromechanical properties of P(VDF-TrFE) exploited in the pro-duction of energy haversters can be used for the development of highly-sensitive sensors and micro-actuators. Examples of such devices are tactile sensor arrays, inexpensive strain gauges, and lightweight audio transducers. PVDF transducers have the advantage of being dynamically more suitable for modal testing than semiconductor piezoresistive transducers, and more compliant for structural in-tegration than piezoceramic transducers. Another feature of this material is the biocompatibility that allows integration of sensors and actuators with human body in biomedical applications.

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27

Chapter 2 Fabrication Methods

In this Chapter, it is described the fabrication of P(VDF-TrFE) samples, anal-ysed in the course of this work. In each section, the technique of deposition and then the technical details of the samples are illustrated. Two kind of samples was analyzed, a spincoated thin film that it is taken as reference for the piezo- and fer-roelectric measurements and different samples with randomly oriented electrospun nanofibers.

2.1 Thin film preparation

The thin film is made of microstructured gratings of an organic ferrolelectric poly-mer and it has been produced at the Max Planck Institute of Mainz from Lenz et al., using a deposition technique called solution micromolding [46]. The procedure is schematically depicted in Figure 2.1a. To allow the fabrication of discrete ca-pacitors, the space between the P(VDF-TrFE) lines has to be backfilled with an electrically insulating polymer. Polymer re-dissolution and stamp swelling have to be avoided, so it has been chosen polyvinyl alcohol (PVA) as the insulating poly-mer, as its solvent, deionized water, neither swells the PDMS stamp nor dissolves P(VDFTrFE).

The film structure is made of copolymer P(VDF-TrFE) (65-35%) that was pur-chased from Solvay. The number- and weight-average molecular weight amounted to 147 and 296 kg mol−1_{, respectively. The average molecular weight of polyvinyl}

alcohol (PVA) (Sigma–Aldrich) was 80–90 kg mol−1_{. P(VDF-TrFE) was dissolved}

in dimethyl sulfoxide (DMSO) and methyl ethyl ketone (MEK). A 400 silicon mas-ter with anti-sticking coating provides a periodic line grating with pitch size of 4 mm, line width of 1.3 mm and height of 2 mm. Stamps complementary to the mas-ter grating were prepared from poly(dimethylsiloxane) (PDMS)and then is casted

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Figure 2.1: a) Schematization of solution micromolding procedure and b) SEM micro-graph micromolded grating(Figure adapted from [46]).

on the master. The stamp/master assembly was evacuated in a desiccator in order to extract air bubbles. After curing for 3 h at 60◦ _{C, the PDMS stamp was peeled}

off from the master. As shown in Figure 2.1a, polymer gratings were fabricated on glass substrates with bottom electrodes of 100 nm thick Au with a 5 nm Cr adhesion layer, prepared by thermal evaporation through a shadow mask. The substrates were subsequently cleaned with UV/ozone and a drop of the polymer solution, P(VDF-TrFE) in DMSO, was put onto the substrate. The PDMS stamp was positioned on top and the substrate/stamp assembly was hot pressed for 2 h. The temperature for P(VDF-TrFE) was set to 140◦ _{C, which is above the Curie}

temperature (120◦ _{C) but below the melting temperature (150}◦ _{C) of the}

copoly-mer. The resulting morphology of the molded gratings is shown in Figure 2.1b (SEM micrograph).

2.2 Electrospun Nanofibers

2.2.1 Electrospinning

Electrospinning process has been known since 1934, thanks to a patent by Formhals consisting in an experimental setup designed for the production of polymer fil-aments using electrostatic force. Therefore, the term electrospinning refers to a process that produces fibers through an electrically charged jet of polymer solution

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2.2. Electrospun Nanofibers 29 or polymer melt. In general, the versatility of electrospinning allows the produc-tion of different polymers, blends, fibers containing precursors, suitable for a wide range of applications. A large number of materials can be directly produced by electrospinning, e.g. polymers and polymer composites, while other materials such as ceramics require post processing of the electrospun fibers. Hence, electrospin-ning is a cheap and simple technique to manufacture nanofibers, thanks to the requirement of common laboratory equipment. However, the science behind this technique is very complex [47].

Figure 2.2: Sketch of the electrospinning process [48]).

Continuous polymeric or inorganic fibers, can be obtained through electrospin-ning by a jet of an electrostatically charged molten polymer or a polymeric solution. Typical dimension of electrospun fibers may range from tens of nanometers to a few microns. The charging process take place in an electrified needle through which the polymer solution flows. The needle is connected to a high voltage DC genera-tor (in the kV range) and a collecting grounded electrode, as shown in Figure 2.2. The polymeric solution, electrostatically charged by the high voltage power supply, comes out from the needle tip in the form of a hanging drop. The high electric field between the needle and a grounded electrode causes a distortion of the drop, until it takes a conical shape, called Taylor Cone. For a critical value of electrostatic potential acting on the charged drop, the resultant force exceeds surface tension and a thin jet of fluid polymer is formed and attracted towards the metal collector.

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Then, the charged jet is accelerated and stretched by the electric field, undergoing to a process of instability, called whipping instability. The filaments run through a spiral path, which increases the stretching process, thus causing thinning of the fiber while the solvent evaporates. During the path, the whipping instability, allows the formation of fibers with diameters in the order of a few hundred nanometers, favoring the evaporation of the solvent and the solidification of the fibers them-selves. The chaotic movement of the jet produces the random deposition of the fibers on the collector. The deposition of aligned fibers is also possible changing the collecting plate with a rotating grounded cylinder, on which the fibers are subjected to an ulterior stretch caused by the rotation [47].

Operational parameters

Some important characteristic parameters of the technique, both related to the polymeric solution and to the electrospinning process, should be pointed out, as they affect the quality of electrospun fibers.

• Surface Tension: the charging process of the polymeric solution has to over-come the surface tension, at the same time a too low value of surface tension may cause a jet breakup in to droplets, leading to a process called electro-spraying [49].

• Polymer Solubility: conductivity and tendency to be polarized affect fiber morphology during the electrospinning process, high dielectric constant sol-vents should be used [50, 51].

• Viscosity: is directly linked to the grade of bonds formed by polymer chains in the solution, so low viscosity can induce electrospraying and polymer particles are formed instead of fibers. On the other hand, increasing the viscosity re-duces the fiber stretching induced by charges and thicker fibers are deposited. Finally, too high viscosity determines problems to pump the solution through the needle or solution drying creates troubles on the needle tip [50, 52]. • Polymer molecular weight: fiber formation occurs only if the molecular weight

of the polymer is sufficiently high to give enough viscosity to the solution. • Solvent evaporation rate: evaporation of the solvent, too quick or too slow,

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2.2. Electrospun Nanofibers 31 formation or fibers still wet. Enviroment condition like humidity and tem-perature affect this parameter.

• Solution conductivity: Accumulation of enough charges has to occur in the solution, in order to increase the repulsive forces, thus overcoming the surface tension of the solution. Subsequent stretching of the jet is connected to the ability of the solution to carry charges. The electric conductivity of solvents is commonly very low, due to the presence of very few free ions. A strategy to increase the electrical conductivity of the solution is the addition of a small quantity of a polar non-solvent of the polymer or proper salts. However, the interaction between solvent mixtures can affect polymer solubility, modifying fiber morphology [51].

• Voltage bias: provides the stretching of the solution, thanks to the columbic force in the jet and the high electric field. Voltage bias also defines the flight time of the jet, governing the stretching force and consequently fiber diameter. Thinner fiber diameters have been observed increasing voltage bias [50]. Higher voltage bias can also increase the degree of crystallinity of the fibers [53].

• Flow rate: this parameter controls how much solution is pushed out from the needle and permits to obtain a stable Taylor Cone, for a given voltage bias. The higher the flow rate, the larger the fiber diameter, due to a bigger volume of solution spun [54].

• Needle diameter: the decrease of the inner diameter of the spinneret causes a reduction in fiber diameter; a drop of solution cannot flow through too small needles [55].

• Needle-to-collector distance: this parameter is very useful to adjust other already cited feature of electrospinning, because affect electric field strength, flight time and solvent evaration rate. Too low distance can cause instability in the process [54].

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Figure 2.3: X-ray diffraction pattern, demonstrating the presence of ferroelectric β phase in the electrospun P(VDF-TrFE) nanofibers[47]).

2.2.2 P(VDF-TrFE) nanofibers

The P(VDF-TrFE) nanofibers that will be analysed in this work, were deposited with an electrospinning process. The same copolymer of the thin film, P(VDF-TrFE)(65/35) furnished by Solvay, was dissolved in a solution of acetone (70% vol.) and dimethyl-sulfoxide (DMSO) (30% vol.). The electrospinning process was made with a classical apparatus for the random deposition of fibers at room condition. The regulation of the flow rate was made by the use of a pump that control the flux of solution through the needle, and was set at 1 ml/h. The grounded metal plate was positioned at 15 cm of distance from the needle, that was electrified applying ± 20 kV. Nanofibers were electrospun applying positive and negative bias on Au, ITO and Si/SiO2 substrates which were connected to the metal collector.

The duration of the process was manually limited in order to deposit a thin layer of fibers that allows to perform PFM and KPFM measurements. In Figure 2.3

is shown an X-Ray diffraction spectrum of P(VDF-TrFE) electrospun nanofibers. It can be seen the characteristic peak of the ferroelectric β phase around 2θ = 20◦_{. This demonstrate only the presence of the crystal ferroelectric phase, but it}

is necessary to do further measurements to establish and quantify the real piezo-and ferroelectric properties of this samples.

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33

Chapter 3 Scanning Probe Microscopy

techniques

In recent years an increasing number of analytical methods have been introduced for comprehensive analysis at the micron and nanometer dimensions. In partic-ular, great interest has been paid to scanning probe microscopy (SPM), a family of methods allowing visualization of surface structures and examination of their mechanical, electromagnetic, optical, and other properties at such scales. The idea of all SPM techniques is based on the acquisition of information about the inter-action that is established between a sharp tip and the surface of samples. In this Chapter I will illustrate the main techniques used for the microscopic analysis of P(VDF-TrFE) samples. The former, called Piezoresponse Force Microscopy, is an advanced technique for the analysis of piezo- and ferroelectric properties at the nanoscale. The latter, is the well known Kelvin Probe Force Microscopy, that has been used for measurements of surface potentials to investigate the electrostatic behaviour of our samples. In order to give a comprehension of these techniques, in general it has been strictly followed the books of Kalinin et al. [56, 57, 58].

3.1 Piezoresponce Force Microscopy (PFM)

Piezoresponse Force Microscopy is a contact SPM mode that is based on monitor-ing piezoelectric surface displacements induced by the electrically biased probmonitor-ing tip. This method was introduced in 1992 by Guethner and Dransfeld to detect polarized regions in ferroelectric copolymer films and, soon after that, proved to be the most effective approach for the nanoscale study and control of ferroelectric domains in bulk crystals and thin films. PFM is capable of providing a wide range of microscopic information about ferroelectric properties with spatial resolution

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of 10 nm such as, static domain configurations, domain switching behavior, slow relaxation processes, and local hysteresis spectroscopy.

3.1.1 Experimental Setup

Figure 3.1: Typical experimental setup for PFM measurements and details of junction between tip and sample surface, indentation and local electric field.

The standard experimental PFM setup is usually based on a commercial scan-ning probe microscope as the one illustrated in Figure 3.1. The instrument is equipped with a laser detection based system for the measure of cantilever deflec-tion, a conductive probing tip, a function generator and, at least, one lock-in am-plifier1

. For the experiments conducted, it has been used the "Park System NX10" SPM (to have more technical details see [59]). In PFM, a conductive probe makes scans across the surface of a piezo- or ferroelectric sample in contact mode. The conductive cantilever plays the role of the top electrode to provide the localized po-larization field to the sample, while the bottom electrode is generally a conductive substrate. The most frequently used probes are either metal-coated silicon probes

1

For the simultaneous acquisition of both vertical and lateral deflection it is necessary to have two lock-in amplifier.

Electrospun polymer nanofibers for electromechanical transduction investigated by scanning probe microscopy

Alma Mater Studiorum · Università di Bologna

ELECTROSPUN NANOFIBERS FOR

ELECTROMECHANICAL TRANSDUCTION

INVESTIGATED BY SCANNING PROBE

MICROSCOPY

Relatore:

Prof. Beatrice Fraboni

Correlatore:

Dott. Tobias Cramer

Presentata da:

Francesco Calavalle

Sommario

Abstract

Contents

Introduction

Chapter 1

Piezo- and Ferroelectric polymers

in electronic applications

1.1

Background

1.1.1

Dielectric materials

1.1.2

Electric Polarization

1.1.3

Electrostrictive effect

1.2

Theory of Piezo- and Ferroelectric effect

1.2.1

Piezoelectricity

1.2.2

Ferroelectricity

1.3

Electrets

1.3.1

Charge formation in electrets

1.4

Piezo- and Ferroelectric polymers

1.4.1

PVDF

1.4.2

P(VDF-TrFE)

1.5

Applications

1.5.1

Energy harvesting devices

1.5.2

Nonvolatile Memories

1.5.3

Other Applications

Chapter 2

Fabrication Methods

2.1

Thin film preparation

2.2

Electrospun Nanofibers

2.2.1

Electrospinning

2.2.2

P(VDF-TrFE) nanofibers

Chapter 3

Scanning Probe Microscopy

techniques

3.1

Piezoresponce Force Microscopy (PFM)

3.1.1

Experimental Setup